The present invention relates to a semiconductor laser deice and to a method for fabricating the same. More particularly, it relates to an increase in the output of the semiconductor laser device.
Recent years have seen rapid widespread use of DVD (Digital Versatile Disk) devices in the fields of AV (Audio-Video) equipment, PCs (Personal Computers), and the like. In particular, great expectations have been placed on the use of recordable DVD devices (such as DVD-RAM and DVD-R) as large-capacity memory devices embedded in PCs and the like and as post-VTR (Video Tape Recorder) devices.
As pickup light sources for the foregoing DVD devices, red semiconductor lasers at wavelengths in the 650 nm band have been used. With the recent increases in the density and capacity of an optical disk, a pick-up light source capable of performing a particularly high output operation over 80 mw has been in growing demand to allow a higher-speed write operation with respect to the optical disk.
If a semiconductor laser device is increased in output, however, each of the laser facets of semiconductor laser device suffers catastrophic optical damage (hereinafter referred to as COD). The catastrophic optical damage is a degradation phenomenon caused by heat resulting from the absorption of a laser beam in the vicinity of the laser facet of the semiconductor laser device. The resulting heat degrades the portion of a semiconductor layer located in the vicinity of the laser facet. Specifically, the heat reduces the band gap of the portion of the semiconductor layer located in the vicinity of the laser facet and increases the absorption coefficient of the portion of the semiconductor layer located in the vicinity of the laser facet. Consequently, the laser beam is further absorbed in the vicinity of the laser facet.
It has been known that, in preventing COD, preliminary provision of a semiconductor layer having a large band gap and transparent to a laser beam emitted from the semiconductor laser device in a region located in the vicinity of each of the laser facets of the semiconductor laser device, i.e., the formation of a so-called window structure is effective. In particular, the formation of the window structure in a semiconductor laser device outputting a red laser beam exceeding 50 mW is inevitable to ensure the reliability of the semiconductor laser device in use.
Thus far, various methods have been proposed each for fabricating a semiconductor laser device having a window structure. One of the methods uses a phenomenon in which diffused Zn alloys a superlattice in an active layer. For example, Japanese Unexamined Patent Publication No. HEI 11-284280 discloses a method in which a window structure is formed by further forming a group III-V compound semiconductor layer containing Zn at a high concentration (hereinafter referred to as a Zn supply layer) over a region located in the vicinity of each of the laser facets of a semiconductor laser device, causing solid-phase diffusion of Zn from the Zn supply layer, and thereby disordering the active layer in the laser facet region. A method of using ZnO as a Zn diffusion source instead of the Zn supply layer is also disclosed in, e.g., Japanese Unexamined Patent Publication No. HEI 10-290043.
FIG. 10 is a perspective view showing a structure of a conventional semiconductor laser device.
As shown in FIG. 10, a conventional semiconductor laser device 70 has a structure (so-called window structure) comprising laser facet regions 713 and an internal region 712.
The internal region 712 has a multilayer structure composed of: an n-type clad layer 701 made of n-type AlGaInP; a guide layer 702a (with a thickness of 30 nm) made of AlGaInP; an active layer 702 made of a quantum well consisting of a plurality of GaInP layers and a plurality of AlGaInP layers; a guide layer 702b (with a thickness of 30 nm) made of AlGaInP; a first p-type clad layer 703 made of p-type AlGaInP containing Zn as a dopant; a current block layer 704 made of n-type AlGaInP; a second p-type clad layer 705 made of p-type AlGaInP containing Zn as a dopant; and a contact layer 706 made of p-type GaAs containing Zn as a dopant, which are stacked successively on a substrate 700 made of n-type GaAs.
The active layer 702 is composed of a repetition of the structure in which the GaInP layers are sandwiched between the AlGaInP layers.
Each of the laser facet regions 713 has a multilayer structure composed of: the n-type clad layer 701 made of n-type AlGaInP; the guide layer 702a (with a thickness of 30 nm) made of AlGaInP; an alloyed active layer 711 made of alloyed GaInP and AlGaInP; the guide layer 702b (with a thickness of 30 nm) made of AlGaInP; the first p-type clad layer 703 made of p-type AlGaInP containing Zn as a dopant; the current block layer 704 made of n-type AlGaInP; the second p-type clad layer 705 made of p-type AlGaInP containing Zn as a dopant; and the contact layer 706 made of p-type GaAs containing Zn as a dopant, which are stacked successively on the substrate 700 made of n-type GaAs.
An n-side electrode 708 made of a metal (such as an alloy of Au, Ge, or Ni) making an ohmic contact with the n-type GaAs substrate 700 is formed on the lower surface of the n-type GaAs substrate 700. A p-side electrode 709 made of a metal (such as an alloy of Cr, Pt, or Au) making an ohmic contact with the contact layer 706 is formed on the upper surface of the contact layer 706.
The alloyed active layer 711 has been disordered through the solid-phase diffusion of Zn. This increases the band gap of the alloyed active layer 711 and forms a window structure which is transparent to a laser beam emitted from the semiconductor laser device 70.
The formation of the window structure through the diffusion of Zn mentioned above increases the reliability of a semiconductor laser device and provides a semiconductor laser device capable of producing a 50-mW class output.
In either of the cases where the methods disclosed in the foregoing publications are used, thermal treatment should be performed in the steps of causing solid-phase diffusion of Zn from the diffusion source to the active layer and alloying the active layer.
If the thermal treatment is performed in the step of causing solid-phase diffusion of Zn, Zn that has been introduced as a dopant in the first p-type clad layer 703, the second p-type clad layer 705, and the contact layer 706 is diffused not only into the portions of the active layer 702 located in the laser facet regions 713 of the semiconductor laser device 70 but also into the portion of the active layer 702 located in the internal region 712 thereof If a dopant such as Zn is diffused into the active layer 702, a nonradiative recombination center may be formed within the active layer 702 to degrade the characteristics of the semiconductor laser device 70. Otherwise, a crystal defect may be formed within the active layer 702 to reduce the lifespan of the semiconductor laser 70.
The amount of the dopant diffused into the individual semiconductor lasers composing the semiconductor laser device 70 is larger as the dopant concentrations of the first p-type clad layer 703, the second p-type clad layer 705, and the contact layer 706 are higher. As the concentration of Zn as the dopant is higher, the problems of the degraded characteristics, reduced lifespan, and the like of the semiconductor laser device accordingly become more conspicuous. To suppress the diffusion of Zn into the portion of the active layer 702 located in the internal region 712, therefore, the doping concentrations of Zn in the first p-type clad layer 703, the second p-type clad layer 705, and the contact layer 706 are preferably lowered.
However, the doping concentrations of Zn in the first p-type clad layer 703, the second p-type clad layer 705, and the contact layer 706 greatly affect the temperature characteristic of the semiconductor laser device 70. If the doping concentrations of Zn in the first p-type clad layer 703, the second p-type clad layer 705, and the contact layer 706 are lowered as described above, the band offset of the conduction band is reduced between the first p-type clad layer 703 and the active layer 702. This indicates that a sufficiently large band barrier against electrons in the conduction band cannot be formed between the first p-type clad layer 703 and the active layer 702. As a result, electrons overflowing from the active layer 702 to the first p-type clad layer 703 are increased. Even if an injected current is increased, an increase in current component contributing to light emission is reduced and a light output is saturated. The problem is encountered particularly at a high temperature that a high output cannot be produced.
FIG. 11 shows a measurement profile obtained as a result of secondary ion mass spectroscopy (hereinafter referred to as SIMS) performed with respect to the laser facet regions 713 and internal region 712 of the conventional semiconductor laser device 70 having the first p-type clad layer 703 doped with Zn at a high concentration. It is to be noted that the dopant had not been introduced into the active layer 702.
As shown in FIG. 11, Zn was mixed in the portion of the active layer 702 located in the internal region 712 irrespective of the fact the dopant had not been introduced therein intentionally. This is because Zn introduced at a high concentration into the first p-type clad layer 703 was diffused in the thermal treatment step for forming the window structure. Similar dopant diffusion also occurs during the operation of the semiconductor laser device.
The present invention has been achieved to solve the foregoing problems and it is therefore an object of the present invention to provide a semiconductor laser device with high reliability.
A first semiconductor laser device according to the present invention comprises: a semiconductor substrate having a first region and a second region adjacent to the first region; a first active layer formed on the first region and made of a compound semiconductor; a first clad layer formed on the first active layer and made of a compound semiconductor containing a first dopant; and a second active region formed on the second region and made of a compound semiconductor containing a second dopant having a diffusion coefficient with respect to the first active region which is higher than that of the first dopant.
The present invention suppresses the diffusion of the first dopant from the first clad layer into the first active layer. This reduces crystal defects in the first active layer and provides the semiconductor laser device according to the present invention with high reliability.
Preferably, a concentration of the first dopant in the first clad layer is in a range of 5xc3x971017 atoms cmxe2x88x923 to 1xc3x971019 atoms cmxe2x88x923.
Since the present invention suppresses the diffusion of the first dopant from the first clad layer to the first active layer, a doping concentration in the first clad layer can be set to a high value. Accordingly, the concentration of the first dopant can be set to a high value in the range of 5xc3x971017 atoms cmxe2x88x923 to 1xc3x971019 atoms cmxe2x88x923. This suppresses the overflow of electrons and provides a semiconductor laser device capable of performing a high-output operation at a high temperature.
The first active layer may be made of (AlxGa1xe2x88x92x)1xe2x88x92yInyP (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61), the first clad layer may be made of (AlcGa1xe2x88x92c)1xe2x88x92dIndP (0xe2x89xa6cxe2x89xa61, 0xe2x89xa6dxe2x89xa61), and the first dopant may be at least one element selected from the group consisting of Mg, Be, Cd, and Hg.
The first active layer preferably includes two types of layers made of compound semiconductors with different band gaps and alternately stacked and the second active layer is preferably made of alloyed compound semiconductors with different band gaps.
In the arrangement, the first active layer is larger in band gap than the first active layer and becomes transparent to a laser beam emitted from the semiconductor laser device. Consequently, the laser beam is emitted from the first active layer without being absorbed by the second active layer. This suppresses or prevents COD in the semiconductor laser device.
An uppermost portion of the semiconductor substrate is preferably composed of a second clad layer made of a compound semiconductor of a conductivity type opposite to that of the first clad layer and the second clad layer preferably contains a third dopant having a diffusion coefficient with respect to the first active layer which is lower than that of the second dopant.
The arrangement suppresses or prevents the diffusion of the third dopant from the second clad layer into the first active layer.
A second semiconductor laser device according to the present invention comprises: a semiconductor substrate; an active layer formed on the semiconductor substrate and made of a compound semiconductor; and a first clad layer formed on the active layer and made of a compound semiconductor containing a first dopant having a diffusion coefficient with respect to the active region which is lower than that of Zn.
The present invention suppresses the diffusion of the first dopant from the first clad layer into the active layer. This reduces crystal defects in the active layer and provides the semiconductor laser device according to the present invention with high reliability.
Preferably, a concentration of the first dopant in the first clad layer is in a range of 5xc3x971017 atoms cmxe2x88x923 to 1xc3x971019 atoms cmxe2x88x923.
Since the present invention suppresses the diffusion of the dopant from the first clad layer into the active layer, a doping concentration in the first clad layer can be set to a high value. Accordingly, the concentration of the dopant can be set to a high value in the range of 5xc3x971017 atoms cmxe2x88x923 to 1xc3x971019 atoms cmxe2x88x923. This suppresses the overflow of electrons and provides a semiconductor laser device capable of performing a high-output operation at a high temperature.
The active layer may be made of (AlxGa1xe2x88x92x)1xe2x88x92yInyP (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61), the first clad layer may be made of (AlcGa1xe2x88x92c)1xe2x88x92dIndP (0xe2x89xa6cxe2x89xa61, 0xe2x89xa6dxe2x89xa61), and the first dopant may be at least one element selected from the group consisting of Mg, Be, Cd, and Hg.
The active layer preferably has a first region including two types of layers made of compound semiconductors with different band gaps and alternately stacked and a second region adjacent to the first region and made of alloyed compound semiconductors with different band gaps and the second region of the active layer preferably contains a second dopant having a diffusion coefficient with respect to the active layer which is higher than that of the first dopant.
In the arrangement, the second region of the active layer is larger in band gap than the first region of the active layer and becomes transparent to a laser beam emitted from the first region of the active layer. Consequently, the laser beam is emitted from the first region of the active layer without being absorbed by the second region of the active layer. This suppresses or prevents COD in the semiconductor laser device.
An uppermost portion of the semiconductor substrate is preferably composed of a second clad layer made of a compound semiconductor of a conductivity type opposite to that of the first clad layer and the second clad layer preferably contains a third dopant having a diffusion coefficient with respect to the active layer which is lower than that of the first dopant.
The arrangement suppresses or prevents the diffusion of the third dopant from the second clad layer into the first active layer.
A first method for fabricating a semiconductor laser device according to the present invention comprises the steps of: (a) preparing a semiconductor substrate having a first region and a second region adjacent to the first region; (b) depositing an active layer made of a compound semiconductor over the first and second regions; (c) depositing, on the substrate, a first clad layer made of a compound semiconductor containing a first dopant; and (d) diffusing, into a portion of the active layer located in the second region, a second dopant having a diffusion coefficient with respective to the active layer which is higher than that of the first dopant to alloy the portion of the active layer located in the second region.
The present invention suppresses the diffusion of the first dopant from the first clad layer into the portion of the active layer located in the first region. This reduces crystal defects in the portion of the active layer located in the first region. The portion of the active layer located in the second region is larger in band gap than the portion of the active layer located in the first region and becomes transparent to a laser beam emitted from the portion of the active layer located in the first region. Consequently, the laser beam is emitted from the portion of the active layer located in the first region without being absorbed by the portion of the active layer located in the second region. This suppresses or prevents COD in the semiconductor laser device and provides the semiconductor laser device with high reliability.
The first method may further comprise the steps of (e) after the step (c), depositing a current block layer made of a compound semiconductor on the substrate; (f) forming an opening configured as a stripe in the current block layer; and (g) depositing a second clad layer made of a compound semiconductor on the substrate.
The first method may further comprise the steps of: (h) after the step (c), successively depositing, on the substrate, an etching stop layer and a second clad layer made of a compound semiconductor; (i) after the step (d), forming a mask having an opening on the second clad layer; 0) removing a portion of the second clad layer located in the opening by using the mask to expose the etching stop layer in the opening; and (k) forming a current block layer made of a compound semiconductor on the substrate.
A second method for fabricating a semiconductor laser device according to the present invention comprises the steps of: (a) depositing an active layer made of a compound semiconductor on a semiconductor substrate; and (b) depositing, on the substrate, a clad layer made of a compound semiconductor containing a dopant having a diffusion coefficient with respect to the active layer which is lower than that of Zn.
The present invention suppresses the diffusion of the dopant from the clad layer into the active layer. This reduces crystal defects in the active layer and provides the semiconductor laser device according to the present invention with high reliability.